The liner of a missile motor consists of a thin layer of a polymerizable material which serves as a bond between the propellant and insulation. It is most generally formulated to be similar to the binder of the propellant. The liner has proven necessary because the insulations, in use in rocket motors, have produced too weak a bond to the propellant, and if a separation were to take place between the propellant and insulation, a burn-around would occur producing a major increase in burning surface and overpressuring of the motor.
Compounding a liner is complicated. Its composition is generally related to the composition of the propellant. It has the same binder, crosslinking agent, etc. The major differences between liner and propellant are that the liner does not contain an oxidizer (ammonium perchlorate), aluminum or nitroamino explosive compounds. The liner is applied using a "sling" lining apparatus that applies the liner through centrifugal force from a spinning spray head. A desired viscosity and certain flow properties are required of the liner. These properties are achieved through the use of inert fillers having high specific surfaces, such as a carbon black, titanium dioxide, silicon dioxide, etc.
The description of a state-of-the-art liner process which has been used in the fabrication of the Pershing II reveals the complexities of a typical liner process. Installation of the Pershing liner consists of the following, and requires approximately 40 hours to complete the following step of this state-of-the-art process:
(a) The surface of the internal insulator is degreased; PA0 (b) The insulator's surface is buffed, and again degreased; PA0 (c) A primer coat is sprayed onto the buffed insulator's surface; PA0 (d) A barrier coat is applied; PA0 (e) The liner is sprayed onto the barrier coat; PA0 (f) The liner is allowed to undergo polymerization to the B-stage; and, PA0 (g) The propellant is cast onto the liner. PA0 (a) The compositions of conventional liners have been such that they undergo a reduction in peel strength as the cure process progressed with aging. PA0 (b) Unnecessarily thick liners have been necessary because the thicker the liner, the better the insulator-to-liner bond has been.
It is recognized that the cost to complete the above process steps is significant, and simplification or elimination of the process steps would offer additional advantages.
The elimination of the need for a conventional-type liner provides a major cost advantage in addition to eliminating severe limitations that the conventional liners have shown to possess. These limitations are discussed below in the following statements relative to the compositions and thickness of the liners:
The peel strength values of most propellants generally undergo a major drop as the liner cure progresses from the minimum value to fully cured. This loss of peel strength with the advancing liner cure is related to the decreasing availability of curing agent to react and form the chemical bond because the curing agent migrates out of the propellant.
The goal is to achieve a propellant/liner bond strength that (when tested to the ultimate) where the failure will occur in the propellant rather than at the bond interface. As a result, the liner is formulated to have a higher tensile strength than the propellant, and similarly the interface is designed so that its strength is also greater than the propellant strength. Thus, the weak link in the system becomes the tensile or cohesive strength of the propellant and failure occurs in the propellant.
Applicant's method of U.S. Pat. No. 5,085,725 not only shortened the processing time for a current liner process of about 39 hours duration (for a Pershing II type missile to about 2 hours), but it also eliminated the need for a liner since the method comprised chemical bonding an isocyanate curable solid propellant composition to the outer surface of the internal insulation of an interceptor solid rocket motor. Specially, the trimer of 1,6-hexanediisocyanate was applied to the internal insulation outer surface and isocyanate curable propellant was placed on the trimer to form a chemical bond between the propellant composition and the internal insulation thereby eliminating the need for a liner between the internal insulation and the propellant composition.
An object of this invention is to provide a method of providing a very strong mechanical interlock between the insulation in a composite interceptor motor and the propellant, and, at the same time, eliminating the need for a liner whose function is to adhesively bond the propellant to the insulation.
A further object of this invention is a method which provides for pre-bonding or pre-vulcanization of a fibrous cloth mat to the insulation prior to motor manufacture.
Still a further object of this invention is to provide a method wherein propellant is cast against the fibrous cloth directly to form a mechanical bond which is reinforced by the fibers of the cloth.